1. Field of the Invention
The present invention relates to a pneumatic tire provided with a main groove extending along a tire circumferential direction, and a land portion comparted by the main groove, on a tread surface.
2. Description of the Related Art
In the circumstances that a consciousness relating to a global environment protection rises in recent years, a rolling resistance of a tire greatly contributes to a mileage performance of a vehicle, and it is necessary to effectively reduce this. Conventionally, there has been proposed a method of reducing the rolling resistance by changing a composition of a tread rubber, however, since it affects a wear resistance and a motion performance of the tire to no small extent, a method which can reduce the rolling resistance independently from the rubber composition is strongly desired.
In order to reduce the rolling resistance, it is important to suppress an energy loss at a time of rolling the tire, and a portion dominating the energy loss is mainly a tread portion. The present inventors have made an intense study, and have found that a strain of the tread rubber caused by a wiping deformation and a deformation of a tire annulus ring greatly takes a part in the energy loss in the tread portion, and an increase of the strain deteriorates the rolling resistance.
In general, when the tire is grounded, a force (also called as an in-plane contractive force) heading for a center portion is applied to an inner side of a ground surface, and the land portion of the tread surface deforms along a tire width direction. In the present specification, such a deformation of the land portion is called as a wiping deformation. The wiping deformation is generated in a whole of the ground surface, however, a great strain is particularly generated in the rubber in the periphery of a groove bottom of the main groove. Further, at a time of grounding, a lower portion of a tire T is pressed against a road surface G as shown in FIG. 5, the annular ring deforms rapidly in circumferential end portions 21 and 22 of the ground surface, and a local strain is generated in the rubber of the circumferential end portions 21 and 22.
In Japanese Unexamined Patent Publication No. 2006-103397, there is described a pneumatic tire in which a reinforcing layer is arranged in an outer periphery of a belt layer. The reinforcing layer is constructed by laminating two reinforcing plies each including a reinforcing element which is inclined at an angle between 5 and 30 degrees with respect to the tire circumferential direction. Further, in Japanese Unexamined Patent Publication No. 2007-1401, there is described a pneumatic tire in which a belt cover layer obtained by winding a reinforcing cord in a tire circumferential direction is arranged in an outer periphery of the belt layer. The belt cover layer is constructed by a belt full cover layer laminated in an outer periphery of the belt layer, and a split belt full cover layer laminated in an outer periphery thereof.
However, since the reinforcing element and the reinforcing cord extend approximately along the tire circumferential direction in the tire, the tire widely allows a deformation along the tire width direction of the land portion and greatly affects a rigidity change relating to a deformation in a circumferential end portion of the ground surface, and it is thought that there is hardly obtained an effect of preventing an increase of the strain of the tread rubber caused by the wiping deformation and the deformation of the tire annular ring. In the first place, the reinforcing layer and the belt cover layer mentioned above are structured such as to be laminated in the outer periphery of the belt layer so as to reinforce the belt layer, and can not sufficiently correspond to such a request as to reduce the rolling resistance.